1. Technical Field of the Invention
This invention pertains to logic interface circuits and in particular to those for use in the automotive environment.
2. Description of the Prior Art
Electrical equipment designed to operate in the automotive environment must have a high degree of electrical noise immunity. The automotive DC power source, often designated "A+" and provided by a battery and charging system with a nominal output rating of 13.6 Vdc, varies considerably during vehicle operation and exhibits severe over- and under-voltage transients. For example, during the operation of starting the vehicle, an electrical starter motor "cranks" the gasoline engine. The heavy current drain, coupled with the internal resistance of the battery and the starting circuit wiring, causes the A+ voltage to drop by several volts and remain low for the duration of cranking Once the engine starts, disengagement of the starter load allows the A+ voltage to return rapidly to its nominal value; charging may then raise the voltage to an upper limit, typically 16.3 Vdc. During abnormal conditions, such as when accessory loads suddenly become disconnected, the A+ voltage may show overvoltage transients. Automotive electrical equipment must typically withstand at least 32 Vdc.
The voltage transients of the A+ supply can cause false triggering or erroneous operation of logic circuits used, for example, to sense the states of transducer outputs. Furthermore, many transistorized circuits and, in particular, commercially available monolithic integrated logic circuit families, often cannot withstand the high overvoltage transients.
Another challenge in the automotive environment has been to economize in electrical equipment cabling and interconnection requirements. Increased complexity and control requirements make it necessary to provide more interconnections between user-interface devices and remotely located electrical systems being controlled. An example of this is a mobile two-way radio installation, which may include both an operator interface in the form of a control unit containing operator controls and a remotely located, vehicle-mounted transceiver unit containing the bulk of the radio circuitry. To reduce cabling density, modern systems often multiplex signals on the interconnection wires.
For many situations, of which control of mobile radios or radiotelephones in automotive environments is merely an example, the prior art has not produced logic interface circuits to meet the difficult requirements of operating over a wide range of supply voltages, withstanding severe transient voltages without failure or erroneous operation, and providing for multiplexing techniques to reduce cost, weight, and space of interconnections.